Bodily soft tissue is remarkably flexible and needs generally to be flexible. It is difficult for a metal stent to match bodily tissue for flexibility. WO 01/32102 is concerned with flexibility of the stent during its journey, from outside the body to the operational site within the body, as for example the catheter delivery system advances along a tortuous path from the point of entry in the body. However, there is also a need for metal stents (and other prostheses) that are destined to be installed at a location in the body where severe bending is to be expected. If the prosthesis could be made more tolerant of severe bending after placement, that would be attractive to doctors and their patients.
WO 01/32102 shows what could be termed a “classic” self-expanding stent structure of zig-zag stenting rings composed of struts interspersed by points of inflection and with adjacent stenting rings linked axially by connector portions. In the compact delivery disposition of the stent (FIG. 3 of WO 01/32102) the struts of the zig-zag rings are more or less parallel to each other so that each point of inflection represents a change of direction for the material of the stenting ring of more or less 180°. As the stent expands to its deployed configuration (FIG. 4 of WO 01/32102) the radius of the stenting ring expands by movement of the struts away from each other so that gaps open up between adjacent struts, and the adjacent points of inflection move further apart (but nevertheless remain spaced at equal intervals to each other around the circumference of the stenting ring).
The connectors serve to restrain relative circumferential movement of the zig-zag rings relative to each other. Thus, if the points of inflection of adjacent stenting rings are facing each other in the compact delivery disposition of the stent, as in WO 01/32102, then so will be these points of inflection still facing each other in the expanded disposition of the stent. When an expanded stent is subject, in the body, to extreme bending, so that the longitudinal axis of the stent is no longer a straight line but a pronounced curve (as in a banana), then the facing points of inflection on the inside of the bend approach each other. The more extreme the bending, the closer the facing points of inflection become until, in the end, these facing “peaks” about each other or rub past each other.
Any such abutment or rubbing is undesirable. One way to guard against it is to choose a stent design that can be classified as a “peak-to-valley” design rather than a “peak-to peak” design as seen in applicant's WO 01/32102. The art is replete with suggestions for peak to-valley designs in which the peaks of any one zig-zag stenting ring do not face corresponding peaks of the next adjacent stenting ring but, instead, are circumferentially offset to the peaks of the next adjacent stenting ring by half of the gap between two adjacent peaks of the same ring, in the expanded disposition of the stent. Then, under extreme bending, any particular peak on the inside of the bend can advance into the V-shaped space between two peaks of the next adjacent stenting ring, without any abutment or rubbing on any portion of the material of the next adjacent stenting ring.
The present invention is concerned with the above-explained problem. It seeks to improve the in situ bend capability of stents including those seen in WO 01/32102. However, the invention also seeks to achieve this performance enhancement without sacrificing other attractive qualities of stents such as disclosed in WO 01/32102. For example, simplicity of modelling of stress distributions within the stent is attractive in the management of fatigue performance of metal stents. Manufacturing simplicity of course facilitates management of cost which should improve access to stents, for those people who need them.
Looking at WO 01/32102, one can quickly see that performance in extreme bending could be enhanced by extending the length of the portions that connect adjacent zig-zag stenting rings. The longer these “bridges” then the more the stent can bend without abutment of peaks on the inside of the bend. However, large axial gaps between adjacent stenting rings are not desirable, for then the tissue of the lumen that is being stented by the prosthesis is relatively unsupported, at least between adjacent stenting rings.
For the same reason, one seeks as far as possible to distribute the metallic material of the stent as evenly as possible over the length and circumference of the tubular envelope of the stent so that support for the stented tissue is as even and uninterrupted as possible. This of course indicates that all gaps between all adjacent struts and points of inflection of the stent matrix should be as constant as possible. Generally, one does not modulate the strut matrix of a stent, over the length and circumference of the stent, to suit tissue variations within the stenting site. Generally, one does not seek to place a particular point on the circumference of the stent in opposition to a particular zone of tissue within the stenting site. For these reasons alone, one seeks a stent matrix that is everywhere the same over the length and circumference of the prosthesis.
In principle, the problem addressed in the present invention, and the solution here disclosed, is useful in all classes of stenting prostheses. That is to say, it works with a stent matrix that exhibits the (spiral) turns around the axis of a helical stent, as well as with a stent that features a stack of endless stenting rings. It works with balloon-expandable stents and resilient self-expanding stents as well as with nickel titanium shape memory alloy stents.